Directional Solidification Casting Research Articles

Thermomechanical fatigue of round tube specimens manufactured by precision directional solidification casting method

AbstractIn the present study, a superalloy round tube specimen prepared by directional precision casting without mechanical turning is experimentally investigated under the thermomechanical fatigue conditions of the in‐phase and out‐of‐phase thermomechanical angels from 500°C to 1000°C. The deformation behavior and damage mechanism of the round tube specimens under TMF load are discussed. It was confirmed that the tension–compression asymmetry under TMF cycling is affected by temperature–load phasing and mechanical strain range. Moreover, the damage to round tube specimens includes creep, fatigue and oxidation. Due to the concentrated stress, the rough surface of the specimen provides suitable locations for crack initiation. A novel life prediction approach that combines the Neu–Sehitoglu model and modified Manson–Coffin model was proposed in response to the traditional life model's insufficient prediction performance. This method may explain the TMF life of precision cast specimens.

Numerical Simulation of Transport Phenomena in Directional Solidification Castings with Changeable Cross-Section and Solidification Interface Control

The roles of traveling magnetic fields (TMFs) within the transport phenomena during the directional solidification of nickel-based superalloys were simulated. The evolution of thermal field, flow field and solid-liquid interface morphology during the solidification process under both natural and forced convection conditions were also simulated and compared. The strength of TMFs window that suppresses the flow of the interfacial front in the melt was quantified. The association between flow velocity at the interface front and defect formation was discussed.

Suppression of liquation cracking susceptibility via pre-weld heat treatment for manufacturing of CM247LC superalloy turbine blade welds

In this study, the weldability of as-cast CM247LC superalloy was metallurgically evaluated for turbine blade applications in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone of the gas tungsten arc welds, and the cracks were characterized as liquation cracks. Metallurgical solutions to suppress the liquation cracking susceptibility were derived via visualization-based spot-Varestraint test. The alloy subjected to aging treatment exhibited the lowest liquation cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC welds were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC could be explained as follows: (i) reduced MC-type carbide, which lowered the local solidus temperature compared with the equilibrium solidus temperature of CM247LC and (ii) increased solidus temperature owing to γ’ precipitation γ’ within the γ matrix. To validate the suppressed liquation cracking susceptibility of the aged CM247LC specimen in real welding, it was subjected to gas tungsten arc welding. The results indicated considerable suppression of liquation cracking compared with as-cast welds; in particular, the reduced liquation cracking temperature range was well-reflected in the welding.

Evaluation of Liquation Cracking Behavior and Susceptibility in Heat-Affected Zone of CM247LC Superalloy Welds for Turbine Blade Application

In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications was metallurgically evaluated in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone (HAZ) of gas tungsten arc welds, and the cracks were characterized as liquation cracks, since a cobble or dropletshaped crack surface consistent with a liquid film was clearly confirmed. Microstructural analysis of the cracking surface and thermodynamic calculations helped elucidate the metallurgical mechanisms of the liquation cracking. In other words, the cracking was attributed to liquation of the γ-γ’ eutectic colony and the constitutional liquation of the MC-type carbides: these phases existed in the as-cast microstructure. In particular, it was calculated that liquation of the γ-γ’ eutectic colony during welding occurs at least at 1488 K and that constitutional liquation of MC-type carbides begins at 1411 K, while the equilibrium solidus temperature of the CM247LC alloy is 1530 K. Finally, the liquation cracking susceptibility was quantitatively evaluated through a spot-Varestraint test, and it was confirmed for the first time that the higher susceptibility of as-cast samples can be suppressed by employing a pre-weld heat treatment such as solution treatment.

Directional solidification casting technology of heavy-duty gas turbine blade with liquid metal cooling (LMC) process

In this work, some important factors such as ceramic shell strength, heat preservation temperature, standing time and withdrawal rate, which influence the formability of directionally solidified large-size blades of heavy-duty gas turbine with the liquid metal cooling (LMC) process, were studied through the method of microstructure analysis combining. The results show that the ceramic shell with medium strength (the high temperature flexural strength is 8 MPa, the flexural strength after thermal shock resistance is 12 MPa and the residual flexural strength is 20 MPa) can prevent the rupture and runout of the blade. The appropriate temperature (1,520 °C for upper region and 1,500 °C for lower region) of the heating furnace can eliminate the wide-angle grain boundary, the deviation of grain and the run-out caused by the shell crack. The holding time after pouring (3-5 min) can promote the growth of competitive grains and avoid a great deviation of columnar grains along the crystal orientation , resulting in a straight and uniform grain structure. In addition, to avoid the formation of wrinkles and to ensure a smooth blade surface, the withdrawal rate should be no greater than the growth rate of grain. It is also found that the dendritic space of the blade decreases with the rise of solidification rate, and increases with the enlarging distance between the solidification position and the chill plate.

Crystal orientation effect and multi-fidelity optimization of a solid single crystal superalloy turbine blade

Single crystal (SC) turbine blade adopts directional solidification casting, traditional design approach requires the [0 0 1] crystal orientation coincide with the blade stacking axis, and the principal deviation angle is less than 10 deg, the other crystal orientation is random. This design philosophy result in random fatigue life of the formed blade even in the same service environment. Crystal orientation effect on mechanical response of a SC turbine blade is investigated using finite element calculation, then presents a multi-fidelity optimization procedure for crystal orientation optimizations of the blade. A nonlinear turbine blade FEM model with orientation dependent crystal plastic theory and coupled temperature conversion provides the high-fidelity model, the low-fidelity model involves a surrogate model technique. Through the multi-fidelity optimization of crystal orientation design, taking into account the efficiency and accuracy to reduce the maximum resolved shear stress (Rss) at the dangerous position of solid turbine blade and effectively improve the fatigue life of the blades. The results demonstrate that crystal orientation have an appreciable variation on Rss in the dangerous position of solid turbine blade. The influence of temperature and structure must be taken into consideration, especially the maximum Rss has the significant effect on evaluating the fatigue life of turbine blade. The optimization model based on the approximation theory can accurately describe the response between the design variables and optimization targets. The optimization reduces the Rss and deformation by 28% and 17%, respectively. The multi-fidelity optimization strategy can effectively optimizes the crystal orientation of the blade and improves the blade life.

Revealing the heterogeneous nucleation behavior of equiaxed grains of inoculated Al alloys during directional solidification

An in-situ study on the directional solidification of an inoculated Al-20 wt%Cu alloy under well-controlled constant cooling rates and temperature gradients has been carried out using a microfocus X-radiography set-up. The influences of temperature gradient and cooling rate on the heterogeneous nucleation rate and growth kinetics of equiaxed grains have been studied quantitatively. It is shown that under the same cooling rate, the nucleation rate of grains decreases with increasing temperature gradient. A high temperature gradient also promotes preferential growth of dendrite arms along the temperature gradient direction, and therefore the formation of elongated grains. However, the temperature gradient effects on nucleation and grain growth decrease with increasing cooling rate. It is revealed that the propagation velocity of the nucleation front in directional solidification castings is approximately equal to the ratio between cooling rate Ṫ and temperature gradient G. Based on the experimental observations, a novel numerical grain size prediction model has been proposed, in which the temperature gradient effect on the nucleation kinetics was rigorously treated by introducing two new concepts termed as ‘inhibited nucleation zone’ (INZ) and ‘active nucleation zone’ (ANZ). The model has been applied to simulate the present in-situ solidification experiments. A good agreement was achieved between the predicted grain number density and the experimental measurements, showing the importance of including the temperature gradient effect on heterogeneous nucleation. Furthermore, the present model also has the capability to predict the temperature gradient necessary for the transition from equiaxed to columnar grain growth.

High-temperature wettability and interactions between Hf-containing NbSi-based alloys and Y2O3 ceramics with various microstructures

To obtain improved mould shells with appropriate microstructures for directional solidification casting of NbSi-based alloys, Y2O3 ceramics with various microstructures were designed and characterized. The mechanisms of high-temperature wettability and interactions between NbSi-based alloys and Y2O3 ceramics were studied. The results showed that there was a characteristic transition in the wettability between the molten alloys and ceramics when the Y2O3 microstructure changed. The spreading rate gradually decreased and the apparent equilibrium contact angle changed from 70.2° to 112.6° with the level of open porosity of Y2O3 ceramics increasing from 3.6% to 27.2%. Wetting kinetics analyses indicated that the movement of the triple line during the wetting process was driven by the formation of continuous HfO2 reaction layer in the alloy-ceramic interface. The content of HfO2 particles inner the alloy matrix with the level of open porosity of Y2O3 ceramics increasing from 3.6% to 27.2% initially gradually decreased and subsequently increased. Y2O3 with open porosity level of about 20.7% were the most beneficial for directional solidification casting of high-purity Hf-containing NbSi-based alloys.

Large-scale simulation of dendrite growth in directional solidification castings (Phys. Status Solidi B 10/2017)

The modified shape function dendritic (MSFD) model was built to simulate the growth of a large number of dendrites in directional solidification (DS) in casting scale. The cover figure (cf. article no. 1600860 by Hang Zhang and Qingyan Xu) shows experimental dendritic morphologies in cross and longitudinal sections fitted by the numerical results with different pre-set Euler angles (φ1, ψ, φ2). The results agreed well, which proved that the transform matrix M was suitable for the dendritic morphology description. For the DS process, an MSFD model coupled with macro models was proposed to simulate the growth of thousands of micro dendrites and their distribution. Furthermore, the crystal selection of the spiral selector is clearly represented by the simulation method that employed the MSFD model, and the findings reveal the distribution of dendrites within the thinnest wall of a single-crystal turbine blade and the solidification sequence of the dendrites. Hence, the modified model coupled with macro-simulation models can be used to predict the morphology and distribution of DS dendrites with respect to large-scale castings produced by the DS process.

Large-scale simulation of dendrite growth in directional solidification castings

In this study, the growth and distribution of directional solidification (DS) dendrites in entire cross-sections of DS superalloy castings are simulated based on a modified shape function dendritic (MSFD) model coupled with the cellular automaton-finite difference (CA-FD) method. The macro-thermal transfer and columnar grain growth are calculated using CA-FD with a macro time-step, and then, the original and boundary conditions for the dendrite-growth simulation are determined via the interpolation of the macro temperature and the projection of the 3D preferential grain orientation using a matrix, M. The dendrite growth calculation cyclically proceeds based on the MSFD model. It is used to predict the growth of hundreds of dendrites throughout the cross-section of a DS casting by considering the macro-solidification parameters. The MSFD model is further applied to predict the competitive growth of DS dendrites and their distribution in the cross-sections of a spiral selector and a fake single crystal turbine blade. The results of the simulation correlate with the results of the DS experiment. Hence, the modified model coupled with the macro-simulation models can be used to predict the morphology and distribution of DS dendrites with respect to large-scale castings produced by the DS process.

Segregation of alloying elements in directionally solidified Re–Ru-containing Ni-based superalloys

In this work, the influence of nonequilibrium conditions of directed solidification was investigated for a nickel-based superalloy containing rhenium and ruthenium. The segregation of alloying elements was researched both as microsegregation into dendrite cell and as macrosegregation along casting. The castings of alloy samples (diameter 20 mm, length 100 mm) were manufactured by slow directional solidification (∼6 mm/h) at a high-temperature gradient (~150°С/cm) by the Bridgeman technique. The alloy research was performed by differential thermal analysis and scanning electron microscopy together with local X-ray spectral analysis. The crystal lattice constants of the γ and γ′ phases were determined by X-ray diffraction analysis at room temperature. Alloying elements such as rhenium and ruthenium are pushed aside into the solid phase; both enrich the dendrite core and initial part of castings. Rhenium and ruthenium also increase the solidus temperature for nickel-based superalloys. On the other hand, alloying elements such as aluminum and tantalum are pushed aside into liquid phase, enriching the interdendritic region and final parts of castings. It is shown that formation of over-alloying local areas in single-crystal castings is a result of microsegregation of alloying elements, mainly rhenium. The over-alloying local areas of single-crystal castings could be a potential reason for formation of TCP phases during heat treatment or long-time high-temperature tests. The γ/γ′-lattice misfit (mismatch of γ-phase and γ′-phase crystal lattices) is not changed in the alloy for directional solidification castings along the full length. This is explained by compensation of the rhenium and ruthenium decrease by the tantalum and aluminum increase along the full length of castings.

Microstructural development and mechanical properties of a near-eutectic directionally solidified Sn–Bi solder alloy

Sn–Bi solders may be applied for electronic applications where low-temperature soldering is required, i.e., sensitive components, step soldering and soldering LEDs. In spite of their potential to cover such applications, the mechanical response of soldered joints of Sn–Bi alloys in some cases does not meet the strength requirements due to inappropriate resulting microstructures. Hence, careful examination and control of as-soldered microstructures become necessary with a view to pre-programming reliable final properties. The present study aims to investigate the effects of solidification thermal parameters (growth rate — VL and cooling rate — ṪL) on the microstructure of the Sn–52wt.%Bi solder solidified under unsteady-state conditions. Samples were obtained by upward directional solidification (DS), followed by characterization through metallography and scanning electron microscopy (SEM). The microstructures are shown to be formed by Sn-rich dendrites decorated with Bi precipitates surrounded by a complex regular eutectic mixture, with alternated Bi-rich and Sn-rich phases. Experimental correlations of primary (λ1), secondary (λ2), tertiary (λ3) dendritic and eutectic spacings (λcoarse and λfine) with cooling rate and growth rate are established. Two ranges of lamellar eutectic sizes were determined, described by two experimental equations λ=1.1 VL−1/2 and λ=0.67 VL−1/2. The onset of tertiary branches within the dendritic array along the Sn–52wt.%Bi alloy DS casting is shown to occur for cooling rates lower than 1.5°C/s.

Erratum to: Characterization of Dendritic Microstructure, Intermetallic Phases, and Hardness of Directionally Solidified Al-Mg and Al-Mg-Si Alloys

Despite the widespread application of Al-Mg-Si alloys, especially in the automotive industry, interrelations of solidification thermal parameters (cooling rate and growth rate), microstructure, and hardness are not properly established. For instance, the control of the scale of the microstructure on both Al-Mg and Al-Mg-Si alloys by adequate pre-programming of the solidification thermal parameters remains a task to be accomplished. In the present study, the directional solidification (DS) of these alloys under unsteady-state solidification conditions is investigated in an attempt to characterize the evolution of microstructural features, macrosegregation, and hardness as a function of local solidification thermal parameters along the DS castings length. Silicon addition to the Al-Mg alloy was found not to affect the sizes of primary and secondary dendrite arm spacings, but induced the onset of tertiary dendritic branches and affected also the size and distribution of intermetallic particles within the interdendritic regions. The Al-Mg-Si alloy is characterized by a more complex arrangement of phases, including binary (α-Al + Mg2Si) and refined ternary (α-Al + Mg2Si + AlFe(Si) eutectic mixtures. As a consequence, a higher Vickers hardness profile is shown to be associated with the ternary Al-Mg-Si alloy DS casting. For both alloys examined, hardness is shown to increase with the increase in the microstructural spacing according to Hall–Petch type equations.

The roles of dendritic spacings and Ag3Sn intermetallics on hardness of the SAC307 solder alloy

Sn–Ag–Cu alloys have emerged as the most promising lead-free solder series among a number of alternatives. These alloys generally present a dendritic Sn-rich matrix surrounded by a eutectic mixture (β+α), where β is the Sn-rich phase and α is the Ag3Sn intermetallic compound. The present study aims to investigate the effects of dendritic (λ2, λ3) and eutectic (λ) spacings and the morphology of Ag3Sn particles on hardness of the Sn–3.0wt%Ag–0.7wt%Cu alloy (SAC307). In order to establish correlations between λ2,3 and hardness, transient directional solidification (DS) experiments were performed permitting a wide range of different microstructures to be examined. The techniques used for microstructure characterization included dissolution of the Sn-rich matrix, optical/scanning electron microscopy. A dendritic microstructure prevailed in the entire DS casting. It is shown that the hardness tends to decrease with the increase in λ2, λ3 and λF (eutectic spacing for Ag3Sn having a fiber morphology). Experimental equations relating microstructural spacings to hardness are proposed.

Microstructural Modeling of Mg Alloys and Experimental Validation

A systematic investigation was carried out in the present study to study the impact of solidification variables like thermal gradient (G), solidification velocity (V) and cooling rate on the second phase fraction and microsegregration in Mg-Al (3, 6 , 9 wt.%) binary alloys. Solidification experiments such as directional solidification and gravity castings were conducted to obtain a cooling rate in the range of 0.05-700 K/s. The experimental results were tested against the microstructural modeling data and a reasonable agreement was obtained between them.

Improving high temperature properties of alumina based ceramic cores containing yttria by vacuum impregnating

Ceramic cores used in investment casting for directional solidification and single crystal blades must have excellent high temperature properties. A strengthening process was developed to improve high temperature properties of alumina based ceramic cores containing yttria. Besides, different parameters, including reinforcing agent, the number of times of vacuum impregnating, were investigated. The experimental results showed that aqueous yttria sol can improve the high temperature properties of ceramic cores through multiple impregnating, which made it a suitable reinforcing agent. Considering the effects of the strengthening process on the apparent porosity and sintered shrinkage of ceramic cores, the optimal vacuum impregnating process was determined. Furthermore, the strengthening mechanism was revealed by using X-ray diffraction and electron spectroscopy analysis. The high temperature properties of the strengthened ceramic cores can meet the requirements of directional solidification and single crystal casting, which benefits the production of turbine blades.

Numerical modeling and simulation of a diffusion-controlled liquid–solid phase change in polycrystalline solids

A fully implicit two-dimensional moving-mesh finite element simulation model was developed to study the influence of grain boundaries in polycrystalline solids on diffusion-controlled liquid–solid transition during transient liquid phase (TLP) bonding. The new model, which was developed without the non-trivial symmetry assumption in existing numerical models for the process, was found to conserve solute and its calculated solutions were unconditionally stable and in good agreement with experimental results. Contrary to the assumption that increased grain boundary diffusion coefficient would significantly accelerate the rate of liquid–solid interface migration, numerical calculations and experimental verification showed that enhanced intergranular diffusivity had a minimal effect on the time required to achieve complete diffusion-induced solidification in cast superalloys. The results indicate that reducing the number of grain boundaries in structural alloys through directional solidification casting techniques did not constitute a disincentive to efficient application of TLP bonding to this class of materials.

Improvement in refractoriness of ceramic shells for directional solidification casting of gas turbine components

Columnar grained and single crystal superalloy blades and vanes for gas turbine applications are made by directional solidification process. In this process, the ceramic mould is subjected to high temperature 1500–1600°C. In order to improve refractoriness of the shell, fine alumina was added to the filler. The fine alumina reacted with silica in the binder forms mullite. The kinetics of formation of mullite and its effects on improvement of refractoriness was studied. Refractoriness is also improved by reducing the amount of sodium in the binder system which is used as a stabiliser. Binders with sodium 0·43 and 0·05% were used for making the shells. This paper would discuss improvement in the shell refractoriness and dimensional stability of the castings of gas turbine components made from Ni base superalloys by directional solidification process.

Influence of solidification rates on a Directional Solidification process for the production of Functionally Graded Materials

This paper is concerned in assessing the possibility of the use of a Directional Solidification (DS) process in order to produce functionally graded materials. It will be assessed the influence of the relative displacement rate between heating coil and mould, in order to evaluate the produced cooling rates along the casting and consequent metallurgical and mechanical properties on the obtained component. A copper-silver alloy, which is prompt to macro-segregation occurrence, will be used. Results demonstrate that the process, which is able to originate DS castings, may also be used for FGMs production.

The Effect of Fluid Convection on Microstructures of Directionally Solidified Castings

Fluid convection during solidification of castings will influence the final structures of the castings. A vertical cylindrical casting set-up has been designed to provide two zones of solidification simultaneously with two moving fronts directionally upward and directionally downward, respectively. This two zone directional solidification casting was composed of a cylindrical Furan sand mold with a water chilled center hole block at the center of the casting. Experimentally it was observed that the cooling rates at different locations of the downward zone were much more even than those of the upward one and this is considered to be the result of a more even temperature distribution along the downward front moving zone due to stronger fluid convection. Fluid convection tends to mix up the liquid and results in a more even temperature distribution and a lower degree of segregation of the element molybdenum which has a strong tendency to segregate to the carbide in ductile iron. The characteristics of the matrix microstructure and the growing direction of carbide of this directionally solidified ductile iron has been investigated with SEM and EBSD techniques.